In the production of water, oil and gas from subterranean formations, scale deposits can frequently result in: plugged well bores, plugged well casing perforations, plugged tubing strings, stuck downhole safety valves as well as other valves, stucked downhole pumps and other downhole and surface equipment and lines, scaled formations and fractures in the vicinity of the well. Such scale formation can occur as a result of mixing of incompatible waters in the well, i.e., waters which when mixed produce precipitates, or as a result of temperature and pressure changes and the like in the produced waters during production. Generally, incompatible waters are formed as a consequence of waterflooding, as injected sea water mixes with formation water in the borehole during water breakthrough. The more common concern are scales deposited because of changes in supersaturation or solubility of minerals in the formation or produced waters caused by pressure and temperature changes, or changes in other physical and chemical environments such as gas compositions, ratio of gas/oil/water. Precipitates which are frequently encountered as scale include calcium carbonate, calcium sulfate, barium sulfate, magnesium carbonate, magnesium sulfate, and strontium sulfate. The deposition of scale is a very complex crystalline process initiated by a supersaturation-induced nucleation of a precipitate of the mineral, scale ions contact these nuclei and the crystal grows in certain crystalline pattern. The adherence of these mineral crystals unto the formation matrix, perforation, well-bore, tubings and equipment is a not well-understood process but once initiated, appears to be spontaneous as seen by the increasing thickness of the scale deposit and the steady decline in productivity. In some cases, production can be halted when valves and pumps are stuck--creating a potentially dangerous situation.
The squeezing of chemicals for protecting wells, particularly oils wells, is widely practiced. A "squeeze" job might last one to six months depending on the nature of the subterranean formation into which the chemical is squeezed and the rate at which fluids e.g., oil and water, are produced by the well. A formation that has low permeability but high porosity, and from which low rates of oil and water are produced would likely bleed injected chemicals back for a long time. However, a problem arises with highly permeable formations which produce high rates of oil and water. This type of formation retains chemicals for only a short time because they are readily washed out of the permeable zones of the formation by the high volumes of produced fluids.
In an article by Carlberg and Essel entitled, "Strontium Sulfate Scale Control by Inhibitor Squeeze Treatment in the Fateh Field", published in the Journal of Petroleum Technology, in June 1982, there is disclosed a method for inhibiting scale formation in a subterranean limestone formation by injecting an acid form of a polyphosphonate which forms a slightly soluble calcium salt. Calcium ions released on dissolution of some of the limestone (calcium carbonate) rock by the acid precipitates calcium polyphosphonate allowing greater retention in the rock. However, this method does not work in sandstones, because sandstones are not soluble in acids, nor do they form calcium ions even when dissolved.
U.S. Pat. No. 3,827,977, discloses the use of water-insoluble metal salts of relatively low molecular weight polyacrylic acids and/or relatively low molecular weight hydrolyzed polyacrylamides as scale inhibitors placed at or near the bottom of a well or formed in situ in the formation that are slowly released into fluids produced by the well.
U.S. Pat. No. 4,602,683 discloses a scale inhibitor precipitation squeeze method wherein an aqueous scale inhibitor solution containing amine phosphonates including salts thereof, having solubility that significantly decreases with decreasing pH, is injected into a formation near a well and thereafter the pH of the solution is lowered to precipitate the scale inhibitor in the formation.
In Applicant's co-pending application Ser. No. 122,301, filed Nov. 18, 1987, there is disclosed a method for inhibiting scale formation and corrosion in a well penetrating a subterranean formation by trapping microparticles of a cation exchange resin (loaded with the desired multivalent cation) and fixed with a scale and corrosion inhibitor into the interstices of the formation in the vicinity of and surrounding the well without rendering the interstices impermeable to the flow of fluids whereby the scale and corrosion inhibitor subsequently bleeds back into fluids produced by the well from the formation, thereby inhibiting scale formation in the well and corrosion in the production system.
The present invention provides a method for inhibiting scale formation and corrosion in a well penetrating a subterranean formation by trapping microparticles of a basic anion exchange resin fixed with a scale and corrosion inhibitor in the interstices of the formation in the vicinity of and surrounding the well without damaging the formation or hindering the flow of fluid. The scale and corrosion inhibitor subsequently bleeds back into fluids produced by the well from the formation, thereby inhibiting scale formation and corrosion.